feat: enable spatial (map) support in the Vega-Lite writer

[cpsievert]

Summary

Enables the ggsql crate's spatial cargo feature so DRAW spatial map layers render through the Vega-Lite writer. Today the wheel parses and validates spatial queries, but rendering fails with Output generation error: Spatial visualization requires the 'spatial' feature to be enabled — after this change, maps render end to end, including through the render_chart() → Altair → browser-widget path.

Runtime requirement: DuckDB spatial extension

All DRAW spatial queries need DuckDB's spatial extension at runtime — the reader issues LOAD spatial automatically (extent computation uses ST_ functions) but never INSTALL it. So users must run INSTALL spatial once per machine (e.g. via DuckDBReader.execute_sql()); the ~59 MB download is cached per DuckDB version/platform under ~/.duckdb/extensions. Without it, spatial queries fail with a clear SQL error pointing at INSTALL spatial.

With the extension present, verified working:

• Polygon maps and choropleths from WKB geometry columns or the built-in ggsql:world dataset (identity projection, with graticule)
• Named map projections (PROJECT TO mercator, equal_earth, …) via server-side ST_Transform
• Point layers and mixed geometry types in one spatial layer
• Faceted small-multiple maps (each panel fits its own extent)

Binary size impact: negligible

Same crate, same release profile, only the feature toggled: 60,737,888 → 60,871,744 bytes (+134 KB, +0.22%). The feature pulls in only the pure-Rust geozero crate (with-wkb/with-wkt/with-geojson) — no native dependencies (no PROJ/GEOS).

Note: ggsql-r has the same feature list (plus odbc); enabling spatial there is an identical one-line change if parity is wanted, but is out of scope here.

Verification

import ggsql

reader = ggsql.DuckDBReader("duckdb://memory")
reader.execute_sql("INSTALL spatial")   # one-time per machine; ggsql LOADs automatically after that

spec = reader.execute("VISUALISE FROM ggsql:world DRAW spatial")
vl_json = ggsql.VegaLiteWriter().render(spec)        # geoshape layers + fitted projection
chart = ggsql.VegaLiteWriter().render_chart(spec)    # Altair LayerChart, to_dict() round-trips

Tests: uv run pytest tests/ — new TestSpatial class covers Vega-Lite rendering of DRAW spatial (basic and choropleth) plus the render_chart() Altair round-trip. The class fixture INSTALLs the extension (skips when offline); a 10-minute timeout on the CI test step bounds download hangs. Rust checks: cargo fmt -- --check and cargo clippy -- -D warnings are clean.